RFID systems have been actively studied recently. FIG. 1 is a block diagram of a conventional RFID system. The RFID system includes a transponder 10 which is also referred to as an RF tag, an antenna 20 for transmitting/receiving RF signals to/from the transponder 10, and a reader (and/or a writer) 30 for processing signals transmitted/received to/from the antenna 20. The reader 30 is connected to a host 40 for transmitting/receiving signals to/from at least one reader and processing the signals. The host 40 can be connected to the reader 30 through middleware (not shown). In this case, the middleware operates and controls the reader 30. Furthermore, the middleware can collect, arrange and filter data.
The transponder 10 is attached to a to-be-identified object such as a product, an automobile, the human body, an animal or the like and stores data such as identification information and state information of the object. The reader 30 transmits electromagnetic waves through the antenna 20 to activate the transponder 10 and reads data stored in the transponder or records new data in the transponder 10. Data is collected by the host 40 and processed into a required format.
The RFID system can continuously recognize the object and easily confirm whether the object is located in the coverage of the antenna 20, and thus attempts to apply the RFID system to various fields, such as distribution systems, parking control systems, hospitals and so on, have been made.
Since a general RFID system is applied to a wide area, only a single reader cannot cover the whole area and multiple readers operate together. However, when more than two readers simultaneously read a single tag, the tag receives read signals from all the readers and cannot appropriately respond to any reader, and thus all the readers fail in reading the tag.
To solve this collision between readers, a method of sequentially switching multiple readers to avoid a simultaneous operation is used. When a system includes N readers, for example, the N readers can be sequentially operated to avoid collision between readers. In this case, if a minimum read time of each reader is Ti, at least a time
      ∑          i      =      1        N    ⁢      T    i  is required to finish the reading operations of all the readers. That is, a very long time is required for the reading operations of the readers. Furthermore, (N-1) switching operations are required for the sequential operations to result in an increase in a system load. Particularly, when readers are connected to a network, switching of the readers accompanies switching of network connection for data communication, and thus the system load further increases.